OFDM (orthogonal frequency division multiplex) systems or systems derived therefrom, for example COFDM (coded OFDM) systems, are used for data transmission in modern digital technology. In this technology, the digital data stream is converted into complex-valued symbols by mapping prior to emission and split up into a multiplicity of subsignals, each of which is transmitted discretely on a separate carrier. In the case of the DVB-T system, for example, 1705 or 6817 such individual carriers are used. In the receiver, these subinformation items are combined again to form a total information item of the transmitter-end digital data stream. An OFDM system is described in greater detail in Hermann Rohling, Thomas May, Karsten Bruninghaus and Rainer Grunheid, Broad-Band OFDM Radio Transmission for Multimedia Applications, Proceedings of the IEEE, Vol. 87, No. 10, October 1999, pages 1778 ff.
Such OFDM signals do not have a constant envelope since, as a result of the addition of the individual carriers, their phase position depends on the modulating signal and the amplitude is subject to severe fluctuations even during a symbol duration or block duration. In the case of a large number of such individual carriers, the PAPR (peak-to-average power ratio) or the CREST factor may be substantially above the mean signal power and consequently substantially greater than unity. This imposes substantial dynamic requirements on the digital/analog converter or amplifier in the transmitter and on the corresponding components in the receiver since they have to provide an appropriate dynamic reserve accompanied by high linearity. At a low loading level with the mean signal power, the operation of the components is therefore very inefficient. The PAPR factor of an OFDM signal is in general different for every block transmitted, whereas the mean power remains the same. Overloading of the transmitter or receiver components results in substantial signal distortions, which means an increased symbol error rate or bit error rate.
There is therefore a need for conditioning such OFDM signals, in which such overloading or underloading is avoided.